This invention relates to electroexplosive devices, and more particularly to protection of electroexplosive devices from electrostatic and radio frequency (RF) ignition and to a composition and method of making a nonlinear resistive shunt and heat-sink to protect an electroexplosive device.
Electroexplosive devices have been in commercial use at least since 1933 when U.S. Pat. No. 1,924,342 was granted to Harvey B. Alexander for a delay cap in which a bridgewire is heated under the influence of electric current supplied from a suitable source through lead wires. The early electroexplosive devices were not very sensitive in that the bridgewires were so selected that significant current would be required to ignite the explosive material. Otherwise an electrostatic charge caused by atmospheric electricity or electromagnetic radiation could produce enough current in the lead wires to heat the bridgewire sufficiently to ignite the explosive material.
As electroexplosive devices have come into greater use, protective devices have been sought to guard against premature ignition, as evidenced by a U.S. Pat. No. 2,247,384 granted in 1941 to Daniel D. Huyett for a shunt placed across the lead wires until it was time to ignite the device. While one could guard against inadvertently connecting the lead wires to a voltage source, and even against electromagnetic radiation (as by properly shielding the lead wires), one could not so easily guard against an electrostatic charge discharging through the bridgewire or from the bridgewire to the case. A technique which has proven successful has relied upon the fact that electrostatic charges are of much greater voltage than that used for intentional ignition. The technique consists of using a shunt material which is nonlinear in behavior. That is, a high resistance is exhibited at voltages of the magnitude used for intentional ignition, and a low resistance at voltages necessary for an electrostatic discharge in air which produce currents sufficient to ignite the device, as described in U.S. Pat. Nos. 2,408,124 and 2,408,125 granted in 1946 to Haus J. Rolfes.
An electroexplosive device useful in igniting various explosive compositions in many different applications, such as solid propellant rocket motors, bombs, seismic charges, land and sea mines, underwater demolition explosives, jet assisted takeoff (JATO) units and other similar types of units, have been commonly referred to as a squib. Although reference will be made hereinafter to a squib in describing an improved, low cost electroexplosive device for use aboard spacecraft and aircraft, it should be understood that the use of the term "squib" is not to imply that the present invention is to be limited to those applications enumerated above, but is rather to be expressly intended to apply to any electroexplosive device (EED) which comes within the terms and true spirit of the invention as defined by the claims.
EEDs are in general relatively small, ranging from 0.090-inch in diameter and 0.150-inch in length to 0.5-inch in diameter and 1.0-inch in length. There are exceptions (e.g., blasting caps vary in length but are generally 0.25-inch in diameter). The bridgewire circuit is generally insulated from the case of the EED, and when one considers the above dimensions it becomes obvious that the spacing between the bridgewire circuit and the external housing becomes quite small, in the order of 0.050-inch. The combination of small gaps and the sensitiveness of primary high explosives (which are loaded on or in these gaps) create the condition for possible inadvertent spark activation.
This condition is most critical when personnel are handling the EEDs. It has been demonstrated that the human body can accumulate electric charges and potentials large enough to cause spark discharges resulting in EED activation. The spark discharge characteristics of the human body vary depending on the body area from which the sparks are drawn (e.g., the hand, finger, or a metal object in the hand). However, the rate of discharge and total energy available from these areas is sufficient to initiate primary high explosives and pyrotechnic materials. There have been cases of inadvertent activation of EEDs attributable to electrostatic discharges from the human body and other sources.
In evaluating an EED for its susceptibility to static discharges, a number of tests and conditions are applied. Under ideal conditions the human body can develop a potential of approximately 25K volts. The average capacitance of the body is about 500 picofarads. The resistance to a discharge emanating from a finger tip is arbitrarily chosen to be about 5,000 ohms. From this information a general test subjecting EEDs to the discharge of a 500 picofarad capacitor charged to 25K volts within a 5,000 ohm resistor in series is often specified. There are variations to this test in which the capacitance, voltage and/or resistance are varied depending on the capability of the EED being tested. The design goal for an EED is usually to meet the 500 picofarad, 25K volts, 5,000 ohm specification. A variety of techniques have been applied to circumvent static discharges away from the sensitive explosive area of EEDs as noted hereinbefore. One recent approach is to purposely create a very small external spark gap between contact pins and an external housing. This technique has allowed EEDs to meet static discharge tests, but control of the gap is difficult and, more seriously, the gap is exposed to the environment, making it vulnerable to changing conditions (dirt, moisture, atmosphere) which can alter its behavior.
Another more recent approach to avoid electrostatic discharge accidents is the use of a varistor shunt, which is made from nonlinear material that becomes highly conductive when a potential gradient above a certain critical value is applied to it. The shunt is usually installed between the connector contact pins external to the hermetically sealed charge. One prior EED (designed for one ampere and one watt no-fire characteristics) uses silicon carbide particles suspended in a room temperature vulcanizable (RTV) rubber as the shunt. Evaluation of this shunt showed that it became conductive at about 400 volts (at the time of fabrication). The voltage at which conductance occurred was not consistent from unit to unit, and not repeatable for the same unit. Repeated testing of the same unit resulted in the clamp voltage increasing directly with the number of tests. Also, an aging phenomenon appears to take place since units which were two years old required about 700 volts to cause conductance. Each passage of an electrostatic discharge through the shunt caused appreciable carbonization of the RTV, so that after multiple discharges, the shunt performed more like a carbon resistor than a nonlinear resistor.
The bridgewire circuit at the end of the contact pins in the EED can absorb radio frequency energy from, for instance, a nearby radar or radio transmitter. If the electric current from such energy flowing through the bridgewire is sufficiently high, it could activate the EED. Thus the problem is to fabricate a material in contact with the bridgewire within the EED which will absorb sufficiently large amounts of thermal energy developed by random currents, yet its thermal capacity must be small enough that it does not dissipate all the heat generated by the current applied to the bridgewire when it is desired to activate the EED. The best solution has been to machine an aluminum oxide cup (a good thermal conductor and dielectric barrier) into which the terminals and the explosive or pyrotechnic material are placed, and allow it to become the substrate for the bridgewire. The explosive or pyrotechnic material is then pressed into the cup and onto the bridgewire. Due to the high loading pressures required and the build-up of tolerances that occur during machining of the aluminum oxide cup and the body of the EED, the cup (aluminum oxide being very brittle) can develop cracks during the loading, resulting in unpredictable performance, particularly after a long storage time. Further, particles of explosive can migrate into the cracks, causing increased electrostatic instability.